Furnaces are used in many processes, such as smelting of metals and processing of glass. In such furnaces a fuel is supplied and combusted to heat the material being processed and a hot waste flue gas is produced. Many present day furnaces use oxy-fuel firing, that is, the use of a fuel with oxygen, instead of the normal combustion air, to improve energy efficiency and productivity, and to reduce undesired nitrogen-oxide (NOx) emissions in the flue gas. A factor to be considered in such oxy-fuel fired furnaces is the efficiency of the combustion process, since oxygen is relatively costly as compared to combustion air.
A significant fraction of energy is lost in the flue gas of high temperature furnaces, even in those of the oxy-fuel fired type. In some air fired furnace systems, the flue gas is used in a regenerator to preheat the combustion air before it is applied to the furnace to increase combustion efficiency.
Oxygen at ambient temperature is generally used in an oxy-fuel fired furnace due to technical difficulties of handling high temperature oxygen for combustion. Oxygen can be preheated in a metallic recuperator to a preheat temperature of up to about 1300.degree. F. Preheating to higher temperature causes problems and raises concerns for the material of the recuperator tubes. In addition, the economics of using recuperators is not very attractive since only a relatively small amount of the flue gas heat is recovered to preheat the oxygen.
A rapid cycle regenerator is available for air-fuel fired furnaces in which the heated flue gas is used to preheat the air. Since the typical cycle time of a rapid cycle regenerator is less than 2 minutes, the size of the beds is small. However, engineering problems exist to preheating of oxygen using these regenerators. One such problem is that, following the teachings of preheating air, the entire flue gas output of the furnace is passed through the regenerator. When preheating oxygen, as compared to preheating air, the flue gas temperature after the regenerator remains excessively high compared to that of the air fired regenerator. Another problem is the residual oxygen remaining in the regenerator at the end of the oxygen preheating cycle. The volume of residual oxygen may correspond to about 5 to 10% of the oxygen flow volume per preheating cycle. Higher NOx emission as a result of higher oxidant preheat temperature is yet another problem.